1. Field of the Invention:
The present invention relates to cerium-based polishing compositions especially adapted for the polishing of organic glasses, and, more particularly, for the polishing of plastic ophthalmic lenses and plastic sheet material.
By the term "organic glasses" are intended all organic shaped articles requiring polishing for any given end application. These include, for example, synthetic polymers, most typically polycarbonates, in particular polydiallylglycol carbonates in the form of ophthalmic lenses or in the form of sheet material, e.g., automotive windows and windshields.
2. Description of the Prior Art:
At the present time, the organic glasses command an ever-increasing share of the glass market. This increase may be attributed to the fact that the organic glasses possess certain characteristics which the mineral (inorganic) glasses do not have. Their impact resistance makes them a material of primary choice when questions of safety are involved: protective glasses, corrective lenses for children, and the like.
Furthermore, they are much lighter and are well suited for glasses requiring delicate mounting or have large diameters.
The polishing of organic glasses is quite delicate and difficulties therein focus on the fact that they are soft, fragile and very sensitive to scratching. Inadequate polishing results in fine scratches and polishing that is too abrasive may result in course scratches, streaks, or even a haze.
While the polishing composition must make it possible to impart a good surface condition onto the organic glass substrate, it must be borne in mind that economic requirements mandate a polishing efficiency sufficient to keep pace with the rate of industrial production.
Polishing compositions usually consist of an abrasive powder in suspension in an organic or aqueous medium, optionally containing other additives.
Exemplary of such abrasive materials, alumina, tin oxide, or mixtures thereof, are currently widely used. If these oxides are contained in the polishing composition, the polishing efficiency is good, but the quality of the polished finish of the organic glass is mediocre in light of deteriorating transparency caused by the presence of a haze and poor surface conditions due to scratching.
It has been suggested in this art to improve these results by using an oxide of a rare earth, in particular ceric oxide. The polishing of organic glasses using an aqueous suspension of ceric oxide has not to date been satisfactory, as numerous fine scratches remain on the surface of the organic glass due to insufficient polishing.
To obviate the aforementioned disadvantages, in German Pat. No. 2,508,871 a polishing compound having the following composition by weight has been proposed: 30% to 45% cerium oxide having a grain size less than 36 .mu.m, 5% to 15% glycerin and 45% to 60% water.
However, the polishing times required in the use of such composition are longer than those required for polishing with the conventional oxides.
Furthermore, the final polish is not entirely satisfactory, if it is desired to perform certain surface treatments, such as anti-reflection or anti-abrasion treatments, on ophthalmic lenses, which require an excellent quality of the support, as they accentuate the presence of any defect.
To improve the quality of the polish and to reduce the polishing time by 10% relative to that required when using ceric oxide alone, French Pat. No. 2,414,071 describes a polishing material containing ceric oxide in suspension in water and combined with a thickener, such as carboxyethylcellulose, ethylcellulose, methylcellulose, polyvinyl alcohol, sodium polyacrylate, polymethylene oxide, carboxypolymethylene, polyvinylpyrrolidone, casein, gum arabic, and a surface tension reducing agent such as an alkaline alkylaryl sulfonate, lauryl sulfonate, dodecyl sulfonate, dodecyl sulfate, lauryl ether sulfate, dioctyl sulfosuccinate, dihexyl sulfosuccinate, diamyl sulfosuccinate, phosphoric acid polyesters and polysiloxanes.
The surface finish obtained by using the aforementioned composition is satisfactory. However, it remains necessary to further improve the polishing time, as an increase of 10% is still inadequate in view of the present rates of industrial production of ophthalmic lenses.